1. Field of the Invention
The present invention generally relates to rotation control methods and storage apparatuses, and more particularly to a rotation control method for controlling rotation of a recording medium such as an optical disk when recording information on and/or reproducing information from the recording medium, and to a storage apparatus which employs such a rotation control method.
Recording media such as optical disks employ a zone constant angular velocity (ZCAV) system or a zone constant linear velocity (ZCLV) system. When the ZCAV system is employed, it is possible to obtain a high random access performance with respect to the recording medium, but a recording and/or reproducing clock frequency becomes low. For example, a data transfer rate at an inner periphery of the optical disk becomes slow. On the other hand, when the ZCLV system is employed, it is possible to obtain a high data transfer rate, but the random access performance with respect to the recording medium becomes poor because the rotational speed of the recording medium such as the optical disk changes upon access thereto.
2. Description of the Related Art
Recently, due to the popular use of the Internet, there are increased opportunities for users to download data, such as music and video data, via a communication line or the like, and to record the downloaded data in recording media. In such cases, magnetic disks of a hard disk drive (HDD), and removable optical disks such as a digital versatile disk random access memory (DVD-RAM) are used as the recording media.
In the case of the video data, the size of one file is relatively large. In addition, the video data is a continuous data, and it is necessary to continuously and stably maintain the data transfer rate over a predetermined value during recording to or reproduction from the optical disk, in order not to generate an image distortion such as frame dropout.
On the other hand, in personal computers or the like, a disk management area is usually provided in a portion of the optical disk for the purposes of managing programs and files. Accordingly, it becomes necessary to access the disk management area every time an access is made to the program or file. For this reason, in a case where the size of each file is relatively small, the access to the file and the access to the disk management area are repeated, to thereby generate a random access to the optical disk.
Therefore, the required data transfer rate and the required random access performance differ depending on the state of use of the recording medium, such as the kind of data processed, the file size and the usage of the recording medium. Hence, it is conceivable to increase the rotational speed of the recording medium such as the optical disk, depending on the required data transfer rate and random access performance.
On the other hand, in optical disk units, the trend of the development is to increase the storage capacity and to increase the recording and/or reproducing speed. If the recording and/or reproducing speed is increased, the data processing speed can be increased when the user uses the recording medium, thereby enabling a more efficient recording and/or reproduction.
As a method of increasing the recording and/or reproducing speed, it is possible to increase the rotational speed of the recording medium such as the optical disk. As other methods of increasing the recording and/or reproducing speed, it is possible to increase the data recording density or to use a direct overwrite type recording medium. However, in order to enable the use of the existing recording system and recording medium as they are, it is desirable to increase the rotational speed of the recording medium, that is, to increase the rotational speed of a spindle motor which rotates the recording medium such as the optical disk, as a method of increasing the recording and/or reproducing speed.
But when the rotational speed of the optical disk is increased, it becomes necessary to increase an emission power of a laser diode which is used as a light source when carrying out a recording or erasure. In other words, the formation of a recording bit depends on an energy product of the irradiating laser light ((irradiation time)×(irradiation power)), and the linear velocity increases and the irradiation time decreases as the rotational speed increases. For this reason, in order to maintain the same energy product, it is necessary to increase the irradiation power.
On the other hand, when carrying out a reproduction, a frequency of a reproduced signal increases and a reproducing margin for decoding decreases as the rotational speed of the recording medium increases.
Therefore, if the rotational speed of the optical disk is increased to a high speed, there is a possibility that the power of the laser diode will become insufficient and/or the reproducing margin will become insufficient. Although the optical disk unit is generally designed to operate with a sufficiently large margin even when the optical disk undergoes a high-speed rotation, there is a possibility that the required read and/or write performance cannot be maintained during the high-speed rotation, due to inconsistencies among the individual optical disk units, inconsistencies in sensitivities of the individual recording media, deteriorated margins caused by temperature changes, and the like.
In addition, when the optical disk is rotated at the high rotational speed, the effects caused by warp and surface undulations of the optical disk and eccentricity of the optical disk become larger by the amount of increase in the rotational speed. In other words, an acceleration in a focus direction caused by the surface undulations of the disk, and acceleration in a track direction caused by the eccentricity of the optical disk respectively become larger as the rotational speed becomes higher. Consequently, it becomes difficult to stably maintain a focus servo state and a tracking servo state, and the focus and tracking servos become unstable and easily slip out of a locked state.
Hence, when the recording medium such as the optical disk is rotated at a high speed in the conventional storage apparatus, there were problems in that there is a possibility that the read and/or write margin may become insufficient, and that it may become difficult to stably maintain the focus servo state and the tracking servo state.